In the present work, it was investigated how addition of KNO3 or K2O affected the NOx conversion on LSF15–CGO10 (La0.85Sr15FeO3–Ce0.9Gd0.1O1.95) composite electrodes during polarization. The LSF15–CGO10 electrodes were part of a porous 11-layer cell stack with alternating layers of LSF15–CGO10 electrodes and CGO10 electrolyte. The KNO3 was added to the electrodes by impregnation and kept either as KNO3 in the electrode or thermally decomposed into K2O before testing. The cell stacks were tested in the temperature range 300–500 °C in 1,000 ppm NO, 10% O2, and 1,000 ppm NO+10% O2. During testing, the cells were characterized by electrochemical impedance spectroscopy, and the NO conversion was measured during polarization at −3 V for 2 h. The concentration of NO and NO2 was monitored by a chemiluminescence detector, while the concentration of O2, N2, and N2O was detected on a mass spectrometer. A significant effect of impregnation with KNO3 or K2O on the NOx conversion was observed. In 1,000 ppm NO, both impregnations caused an increased conversion of NO into N2 in the temperature range of 300–400 °C with a current efficiency up to 73%. In 1,000 ppm NO+10% O2, no formation of N2 was observed during polarization, but the impregnations altered the conversion between NO and NO2 on the electrodes. Both impregnations caused increased degradation of the cell stack, but the exact cause of the degradation has not been identified yet.